The rapidly growing demand for mobile communications equipment has spurred intensive research and development work on power amplifiers used in communications equipment. Semiconductor transistors utilized in mobile communications equipment power amplifiers include heterojunction bipolar transistors (hereafter abbreviated to HBT), field effect transistors (hereafter abbreviated to FET), and SiMOS (Metal-Oxide-Semiconductors) FET, etc. Among these semiconductor transistors, the HBT possesses features including excellent linear input/output characteristics, operation only on a positive supply voltage, not requiring a negative power generation circuit and components, and a high output power density, as well as a low cost and compact space-saving size due to a small chip surface area. The HBT is therefore a main type transistor oriented to mobile communications equipment for power amplifiers.
However, the HBT has the problem of unstable operation due to heat in a phenomenon called thermal runaway. To deal with this problem, a stabilizing resistor called a ballast resistor is used to stabilize the operation. One means stable operation via a ballast resistance utilized an AlGaAs ballast resistor layer over the InGaP emitter layer. This technology was disclosed in the re-publication of WO98/53502, Table 1 (patent document 1).    [Patent document 1] WO98/53502 (Table 1)
The technology disclosed in WO98/53502 had the problem of reliability in electrical conduction to the device. In other words, a drastic deterioration in device characteristics occurred in HBT containing an AlGaAs ballast resistor layer on top of an InGaP emitter layer. On the other hand, the problem of unstable operation due to thermal runaway as described above occurred when this type of ballast layer was not used.